Known devices of the aforesaid type comprise electromagnetic proximity sensors. These sensors, each fixed into a suitable seat, comprise a first part with an axially movable rod, to the free end of which there is secured an idle round element. This round element is in contact with a flat lateral face of the movable annular welding member. The sensor also comprises a second part fixed in a suitable seat provided in the face of the annular member, on which face it slides together with the round element of the first part.
The second part is shaped as a small flat raised piece having its end portions inclined to smoothly join the face.
The raised piece is fixed in correspondence with one or more predetermined welding heads, the first part being fixed in predetermined positions of the frame.
When the annular welding member rotates and the round element of the first part of that sensor makes contact with the raised piece in predetermined angular positions, this causes axial movement of the rod of the first part of the sensor, with consequent sensor excitation.
Proximity sensors of the aforesaid type have been used for more than twenty years in known welding devices, and in spite of their considerable drawbacks an alternative solution has never been found.
The most frequent drawback of the aforesaid sensor is related to the fact that very frequently the second parts of the sensor become separated from the face to which they are secured or on which they move. In this respect the raised piece which forms the second part of the sensor is of relatively small dimensions and is secured to the annular member by a small screw of headless type, the free end of which must be in line with that surface of the raised piece on which the round element of the first part of the sensor makes contact.
It should also be noted that the rod comprising the round element exerts a considerable pressure on the raised pieces, the electromechanical sensors used in known devices being of the type able to be used within a very large temperature range (-40.degree. C. to +150.degree. C.), all the components of the first part of the sensor being suitably dimensioned for this purpose.
Consequently it frequently happens that after traversing the raised pieces a number of times, the rod of the first part detaches them displaces them or causes the screw securing the raised piece to partially emerge, this possibly resulting in an angular shift of the round element such that the sensor can no longer perfectly operate.
In the worst case, that of detachment of the raised piece, that part of the device comprising the welding heads must be completely dismantled, an operation requiring a considerable time during which the device remains inoperative.
The case of displacement of the raised pieces is much more frequent than their detachment. Generally the raised pieces have to be repositioned every 8-10 welds. It should be noted that although the repositioning of the raised pieces is not as complicated as their replacement, it still requires a considerable time as it has to be done with maximum precision.
Taking account of the fact that in a medium sized device five sensors with relative raised pieces are provided, it will be apparent that the problem caused by maloperation of the sensors represents a considerable problem which on the one hand results in long periods of device inoperability and on the other hand requires numerous spare parts to be available, this not being always possible as generally the devices operate in extreme situations and in places much out of the way.
In attempting to solve the aforesaid problems, during the last twenty years it has been sought to improve the characteristics of the screws for fixing the raised piece and to limit the force exerted by the round element on the raised piece. Satisfactory results have not however been achieved up to the present time.